It is known to make a slide-fastener stringer half or a warp-knit tape, a monofilamentary coupling element lying on the edge of the tape and extending slightly transversely therefrom, and stitching securing this coupling element to the tape. Such a stringer half is suitable for use in light-duty and medium-duty slide fasteners. Nonetheless it cannot withstand the considerable transverse and longitudinal stresses which it would be subjected to if used in a heavy-duty slide fastener.
Thus it is standard practice in heavy-duty slide fasteners to use instead of a helicoidal monofilamentary coupling element a succession of discrete metal or synthetic-resin teeth which are secured to a thickened edge of the support tape. To this end the edge of the support tape is formed with a welt or the like so that the teeth or coupling heads can be mounted straddle-fashion on this edge.
When such a support tape is warp knit the customary prodecure is simply to load the endmost needle of the machine with the heaviest possible yarn so that the chain at this edge will be as thick as possible and form the above-described welt. Such a production method places a limit on the thickness which can be imparted to the edge chain in the warp-knit tape as the needle has a limited capacity that can only accpet yarns up to a certain gauge.
For this reason warp-knit support tapes for heavy-duty slide fasteners have not been adequately strong to withstand the considerable transverse pull exerted on the coupling heads of such slide fasteners since the edge welt is not sufficiently large or well integrated with the rest of the tape to form a good attachment for the teeth. Furthermore a warp-knit tape is frequently overly elastic in the longitudinal direction so that it can stretch and change the intertooth spacing of the fastener, allowing the fastener to open accidently.